The present invention relates to a ceramic membrane structure and method of separating oxygen with the use of the ceramic membrane structure. More particularly, the present invention relates to such a ceramic membrane structure and method in which the membrane structure is formed of a mixture of an ionic conducting material and a mixed conducting material. Even more particularly, the present invention relates to such a ceramic membrane structure and method in which the membrane structure is formed by dense and porous supporting layers that are formed from the mixture of materials.
Oxygen-selective ceramic membranes are fabricated from a ceramic that conducts oxygen ions at high temperatures. In such a ceramic membrane, the heated membrane is exposed to an oxygen-containing gas that ionizes at a cathode side of the membrane. Under a driving force of a differential oxygen partial pressure, oxygen ions are transported through the membrane to an opposite anode surface. The oxygen ions combine at the anode surface of the membrane to give up electrons that are transported through the membrane or a separate electronic pathway to ionize the oxygen at the cathode side of the membrane.
The resistance to oxygen ion transport is in part dependent on the thickness of the membrane. Therefore, very thin membranes are desirable. A recent development in ceramic membrane technology is to form a thin dense layer of ion transport material on a porous support. The dense layer conducts oxygen ions and the supporting structure functions as a percolating porous network to add structural support to the dense layer. The porous support may also be fabricated from a material that is itself capable of transporting ions so as to be active in separating the oxygen.
The materials used in forming ceramic membrane can be classified as either mixed conductors that are capable of conducting both the oxygen ions and the electrons required to initially ionize the oxygen or ionic conductors that are capable of conducting only the oxygen ions. Ionic conductors require separate electrical pathways for the conduction of the electrons.
An example of an ionic conductor with separate electrical pathways can be found in U.S. Pat. No. 5,306,411. In this patent, solid membranes are disclosed that comprise a multi-phase mixture of an electronically conductive material such as a noble metal and an ion conductive material for use in oxygen separation from air for electrochemical reactions and applications. The oxygen ion conductor facilitates all the oxygen transport and the electronic conductor does not take part in the oxygen permeation but rather, provides the required electronic pathway for electrons.
In ion conducting materials such as discussed above, a volumetric inefficiency results from the fact that part of the volume of the membrane is taken up with the electronically conductive material that does not take part in ion transport. Mixed conducting materials can therefore be said to be more efficient than ion conducing materials on a volumetric basis. However, the ceramics utilized in mixed conductors present several problems in realizing a multi-layer composite having dense and active supporting layers. One major problem is that such ceramics, commonly oxygen deficient perovskites, are not particularly robust. This problem is addressed in U.S. Pat. No. 5,911,860 which discloses the addition of a metallic or ceramic second phase to a mixed conducting perovskite to improve the mechanical strength of the material and prevent microcracking. The ceramic second phase is not present above the percolation threshold and as such, does not contribute to the separation. Hence, such a material is also not as efficient in terms of oxygen ion transport on a volumetric basis as a membrane formed of the mixed conducting perovskite alone. Moreover, even where a strong material is selected for the porous supporting layer, there can be a thermal incompatibility between the dense layer and the supporting layer which arises from the materials making up such layers having different thermal coefficients of expansion.
It is to be noted that there have been materials fabricated from a mixture of a mixed conducting phase and an ionic conducting phase where the ionic conducting phase is present above the percolation limit. For instance, in V. Dusastre et al., xe2x80x9cOptimization of Composite Cathodes for Intermediate Temperature SOFC Applicationsxe2x80x9d, Solid State Ionics, Vol. 126, p163, a two phase porous material consisting of La0.8Sr0.4Co0.2Fe0.8O3-xcex4 (a mixed conductor) and Ce0.9Gd0.1O2-xcex4 (an ionic conductor) is used to improve the cathodic polarization of an electrode in a fuel cell. In the fuel cell structure disclosed in this article, the electrode is a thin layer of material of about 10-15xcexcm without supporting capability that is used to conduct electrons from or to an external load. The electrolyte itself thus has a higher ionic conductivity than electronic conductivity and is not designed to conduct electrons. Another example of such a two phase mixture can be found in U.S. Pat. No. 5,478,444 which discloses a mixture of an oxygen ion conducting phase such as bismuth or cerium oxides and an electronic conducting phase such as a metal or perovskite. In this patent the oxygen ion conductor facilitates all the oxygen transport and the electronic conductor does not take part in the oxygen permeation.
A further problem with perovskites, is that chemically induced strains may be introduced in a supporting structure fabricated from such materials. As the oxygen partial pressure is reduced in a membrane formed from a perovskite, such as on the anode side thereof, there is an initial expansion followed by a substantial contraction. The contraction is due to the chemical instability of the material. The transition metal cations are reduced and the perovskite structure is no longer maintained. Prolonged exposure to a reduced oxygen partial pressure can produce an irreversible transition. Such problems are exacerbated where the supporting layer is exposed to a fuel gas such as in a reactive purge or in reforming operations because part of the membrane is exposed to the lowest oxygen activity in the membrane coupled with fuel diffusing into the membrane.
As will be discussed, the present invention provides a ceramic membrane structure and a method of separating oxygen in which the membrane structure has a dense layer supported by one or more active porous layers formed of a mixture of a mixed conductor and an ionic conductor that allows for a more efficient oxygen ion transport than membranes of the prior art. Moreover, a membrane of the present invention is more chemically stable, mechanically stronger and has superior creep resistance than membranes employing mixed conductors alone.
The present invention, in one aspect, provides a ceramic membrane structure for separating oxygen from an oxygen containing feed comprising a dense layer and at least one active porous layer. It is to be noted that the term, xe2x80x9coxygen containing feedxe2x80x9d as used herein and in the claims means a gaseous mixture containing oxygen such as air or a gas containing oxygen in a combined form such as water. Furthermore, the term, xe2x80x9cdense layerxe2x80x9d as used herein and in the claims means a layer that is essentially impervious to the passage of the oxygen molecules to be separated from the gaseous mixture as opposed to a porous layer that would permit such passage. Such a layer can be made extremely thin to lessen the degree of resistance to oxygen ion transport through such a layer. A thin dense layer, however, has to be mechanically supported. The requisite support is provided by a porous supporting layer that is active so that it can take part in the oxygen ion transport.
In the present invention, the dense layer contains at least a mixed conducting material and the at least one active porous layer is formed of a mixture having an ion conducting phase capable of predominantly conducting oxygen ions and a mixed conducting phase capable of conducting both said oxygen ions and electrons. The ion conducting phase is present within the mixture in an amount greater than a percolation threshold. The mixed conducting material of the dense layer and the mixed conducting phase of the at least one active porous support have greater electronic conductivity than ionic conductivity.
In a further aspect, the present invention provides a method of separating oxygen from an oxygen containing feed stream. In accordance with such method, the feed stream is introduced to a cathode side of a ceramic membrane element having a membrane structure of the type set forth above. During the introduction of the feed stream to the cathode side of said ceramic membrane, the membrane is maintained at a temperature of at least about 600xc2x0 C. and a pressure difference is maintained across said membrane from said cathode side to an opposite anode side thereof.
In either aspect of the present invention, the ion conducting phase can be formed of a first material having an oxygen ion conductivity greater than about 0.01 S/cm at 1000xc2x0 C. in air and the mixed conducting phase can be formed from a second material having an oxygen ion conductivity of greater than about 0.01 S/cm at 1000xc2x0 C. in air and an electronic conductivity greater than about 0.02 S/cm at 1000xc2x0 C. in air. The first material can be a fluorite, a bismuth oxide, an apatite oxide, and mixtures thereof and the second material can be a perovskite or a brownmillerite. Preferably, one of the first and second materials is present within the mixture in an amount of no less than about 5% by volume and the other of the first and second materials is present within said mixture at no greater than about 95% by volume. More preferably, one of the first and second materials is present within the mixture in an amount of no less than about 10% by volume and the other of the first and second materials is present within said mixture at no greater than about 90% by volume. Most preferably, one of the first and second materials is present within the mixture in an amount of no less than about 20% by volume and the other of the first and second materials is present within said mixture at no greater than about 80% by volume.
In the present invention, the ion conducting phase of the mixture is present above the percolation limit so that not only does it enhance properties such as strength, creep resistance, and chemical stability, but also, it is able to contribute to the ionic conduction within the material. Thus, the mixed conducting phase significantly contributes to the required electronic transport and both the mixed and ionically conducting phases contribute to the oxygen ion transport. This results in a membrane that is more efficient on a volumetric basis than prior art composite membranes in which part of the membrane volume is taken up by materials that do not contribute to oxygen ion conduction. It is to be noted that where the prior art employs a second metallic phase, such second phase is typically a noble metal that increases the cost of the membrane. Moreover, metal/ceramic dual phase materials are also not stable in fuel conditions due to metal segregation and dewetting of the metal. A central advantage of the present invention is that thermal expansions between the dense and active porous layers can be closely matched. For instance, the dense layer can be formed from a perovskite and the porous supporting layer can be formed from a mixture containing about 50% by volume of the perovskite and about 50% by volume of the ion conducting phase. Other combinations are of course possible, for instance, forming both the dense and active porous layers from the mixture of mixed conducting and ionic conducting phases. The most preferred composition of the porous support is dependent on the operating temperature of the membrane. At high operating temperatures the membrane is more active but requires more robust supporting materials. At lower temperatures the process is less active and thus requires more active materials, but does not require as robust a substrate. Therefore at temperatures about 1050xc2x0 C. the most preferred composition may contain about 80% by volume of the oxygen ion conducting phase and about 20% by volume of the mixed conducting phase, while at 750xc2x0 C. the most preferred composition may contain about 20% by volume of the oxygen ion conducting phase and about 80% by volume of the mixed conducting phase.
As mentioned above, single-phase perovskite materials that have sufficient oxygen ion conductivity have high and very non-linear thermal expansion. As will be shown, a material of the present invention has lower thermal and compositional expansions than prior art materials.
A still further advantage of the present invention concerns the fact that the maximum oxygen flux in a membrane is achieved when the ionic transport number (the ratio of the oxygen ion conductivity to the total conductivity) is about 0.5. Typically mixed conducting perovskites have an ionic transport number of 0.01, while the ionic transport number of ion conducting fluorites is greater than about 0.9. Thus adding the two phases brings the overall ionic transport number for the membrane closer to the desired 0.5.